How do I use extern to share variables between source files?

                 declare | define   | initialize |
                ----------------------------------

extern int a;    yes          no           no
-------------
int a = 2019;    yes          yes          yes
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int a;           yes          yes          no
-------------

Declaration won't allocate memory (the variable must be defined for memory allocation) but the definition will. This is just another simple view on the extern keyword since the other answers are really great.

With xc8 you have to be careful about declaring a variable as the same type in each file as you could , erroneously, declare something an int in one file and a char say in another. This could lead to corruption of variables.

This problem was elegantly solved in a microchip forum some 15 years ago /* See "http:www.htsoft.com" / / "forum/all/showflat.php/Cat/0/Number/18766/an/0/page/0#18766"

But this link seems to no longer work...

So I;ll quickly try to explain it; make a file called global.h.

In it declare the following

#ifdef MAIN_C
#define GLOBAL
 /* #warning COMPILING MAIN.C */
#else
#define GLOBAL extern
#endif
GLOBAL unsigned char testing_mode; // example var used in several C files

Now in the file main.c

#define MAIN_C 1
#include "global.h"
#undef MAIN_C

This means in main.c the variable will be declared as an unsigned char.

Now in other files simply including global.h will have it declared as an extern for that file.

extern unsigned char testing_mode;

But it will be correctly declared as an unsigned char.

The old forum post probably explained this a bit more clearly. But this is a real potential gotcha when using a compiler that allows you to declare a variable in one file and then declare it extern as a different type in another. The problems associated with that are if you say declared testing_mode as an int in another file it would think it was a 16 bit var and overwrite some other part of ram, potentially corrupting another variable. Difficult to debug!

A very short solution I use to allow a header file to contain the extern reference or actual implementation of an object. The file that actually contains the object just does #define GLOBAL_FOO_IMPLEMENTATION. Then when I add a new object to this file it shows up in that file also without me having to copy and paste the definition.

I use this pattern across multiple files. So in order to keep things as self contained as possible, I just reuse the single GLOBAL macro in each header. My header looks like this:

//file foo_globals.h
#pragma once  
#include "foo.h"  //contains definition of foo

#ifdef GLOBAL  
#undef GLOBAL  
#endif  

#ifdef GLOBAL_FOO_IMPLEMENTATION  
#define GLOBAL  
#else  
#define GLOBAL extern  
#endif  

GLOBAL Foo foo1;  
GLOBAL Foo foo2;


//file main.cpp
#define GLOBAL_FOO_IMPLEMENTATION
#include "foo_globals.h"

//file uses_extern_foo.cpp
#include "foo_globals.h

In short extern means that variable is defined in other module and its address will be known at link time. The compiler does not reserve memory in current module and knows the variable type. To understand extern is good to have at least little experience with assembler.